double for loop and double if condition in R

I have a list in R which contains 7 dates, and I want to search for those dates in another dataframe and change the value of another column.

My list is ("2015-03-29" "2016-03-27" "2017-03-26" "2018-03-25" "2019-03-31" "2020-03-29" "2021-03-28")

The data frame is:

df

date          time     time_prog    value1   value2
2015-01-01    0:00             1      4532       334      
2015-01-01    0:10             1      4589       485
2015-01-01    0:20             1      4232       324      
2015-01-01    0:30             1      4689       345
2015-01-01    0:40             1      4572       354      
2015-01-01    0:50             1      4129       455
2015-01-01    1:00             2      3332       324      
2015-01-01    1:10             2      3589       475
2015-01-01    1:30             2      5532       384      
2015-01-01    1:40             2      6589       685
2015-01-01    1:50             2      4552       534      
2015-01-01    2:00             3      4549       475
2015-01-01    2:10             3      4232       384      
2015-01-01    2:20             3      4289       455
.
.
.
omitted 331363 rows

What I want is a new column called time_prog2 that changes the value of time_prog by substraining 1 to the value of that column but starting when the column hour is equal to 3:00 and ending at the end of that day.

I tried using a loop but it changes all the values from the data frame until the end and that is not what I want, I just want a column to be changed for all the hours starting in 3:00 and until the end of the day, for 7 certain dates which are in a list. I also tried a loop with 2 conditions (if ... && ...) but it is really long and i left the computer for one hour and it was still running so that can not be the solution. Any idea? Sorry for my English, it is not my native language. Thank you!